1. Field of the Invention
The present invention relates to an image forming apparatus and control method for performing toner supply control using a video count, and a computer-readable medium.
2. Description of the Related Art
A developing device in an image forming apparatus of an electrophotographic type or electrostatic printing type generally adopts a two-component developer mainly containing toner particles and carrier particles. Especially, most developing devices in color image forming apparatuses for forming a full-color or multi-color image by an electrophotographic method use a two-component developer in terms of image tint and the like. The toner density (that is, the ratio of the weight of the toner particles to the total weight of the carrier particles and toner particles) of the two-component developer is a very important factor to stabilize the image quality. The toner particles of the developer are consumed in development and the toner density changes. Thus, the toner density needs to be always controlled constant to maintain the image quality by accurately detecting the toner density of the developer using a developer density control device (ATR: Auto Toner Replenisher), as needed, and supplying toner in accordance with the change.
To correct a change of the toner density in the developing device upon development, various types of toner density detection devices and density control devices in developer containers are in practical use to control the amount of toner to be supplied in development.
An example of these methods is an optical detection method using the fact that when a developer conveyed on a developer carrier or one in a developer container is irradiated with light from the vicinity of the developer carrier or the developer conveyance path of the developer container, the reflectance changes depending on the toner density. Note that the developer carrier is generally a developing sleeve and will be referred to as a “developing sleeve”. There is also proposed an inductance detection method of detecting an actual toner density in a developing unit based on a detection signal from an inductance head which detects an apparent magnetic permeability based on the mixing ratio of a magnetic carrier and non-magnetic toner on side wall of the developer container and converts it into an electrical signal. Toner is supplied in accordance with a comparison between a detected density and a reference value using a developer density control device which detects and controls the toner density by these methods.
Another method is as follows. A patch image formed on an image carrier is irradiated with light emitted by a light source arranged at a position facing the surface of the patch image. A sensor receives the reflected light and reads the density of the patch image. An analog-to-digital converter converts the read value into a digital signal, and sends the digital signal to a CPU. If the read value indicates a density higher than an initial set value, toner supply stops until the density returns to the initial set value. If the density is lower than the initial set value, toner is forcibly supplied until the density returns to the initial set value. As a result, the toner density is indirectly maintained at a desired value. Note that the image carrier is generally a photosensitive drum and will be referred to as a “photosensitive drum”.
However, the method of detecting a toner density from a reflectance obtained when a developer conveyed on the developing sleeve or one in the developer container is irradiated with light has a problem that no toner density can be detected accurately if the detector is contaminated with scattered toner or the like.
The inductance detection ATR has a problem that a sensor detection signal corresponding to an apparent magnetic permeability changes discontinuously as the bulk density of the developer changes when the developer is left to stand or the environment varies immediately before the operation of the image forming apparatus stops or immediately after the operation restarts.
The method of controlling a toner density indirectly from a patch image density has a problem that neither a space large enough to form a patch image nor a space large enough to install a detector can be ensured as a copying machine or image forming apparatus becomes compact.
As a method free from these problems, a toner supply method using a video count has come into practical use (see, for example, Japanese Patent Laid-Open No. 5-323791). In this method, to keep constant the toner density in the developing unit that decreases upon development, the output levels of the digital image signals of respective pixels are accumulated to obtain the printing ratio of an output image. A toner amount to be consumed is calculated from the obtained printing ratio, and toner is supplied in development. More specifically, a video count corresponding to the tone values or dot counts of respective pixels in multi-level video data in image processing, binary video data after halftone processing, or the like is converted into a toner supply amount. The converted toner supply amount is sent to a CPU. The CPU transmits a toner supply signal for a predetermined time based on the toner supply amount. In response to this, a toner supply device is driven to supply a necessary amount of toner to a developer container. Hence, the toner density is kept constant in the developer container.
However, in Japanese Patent Laid-Open No. 5-323791, the value of a toner supply amount that is generated based on a video count value accumulated from video data during image processing is temporarily transmitted to the CPU, and output as a toner supply signal from the CPU, driving the toner supply device. In this case, software processing by the CPU intervenes, so a development timing based on supply target video data does not match an actual toner supply operation timing.
For a tandem engine in which drums for forming an electrostatic latent image are arranged in tandem, multi-level image data undergoes halftone processing and is rasterized into respective binary color component data. Then, the color component data are sequentially transmitted to corresponding drums at timings to form an electrostatic latent image. These timings actually have a time difference based on the distance between the drum stations of respective color components and the printing speed. Actual color toners are sequentially consumed with this time difference. For example, a case in which drum stations are arranged in order of Y, M, C, and K (Yellow, Magenta, Cyan, and Black) will be considered. In this case, when an electrostatic latent image of the first page is formed on the K drum at the final stage, an electrostatic latent image of the second page to be printed next is formed on the Y drum. In control of simultaneously performing Y, M, C, and K toner supply operations, a time difference is always generated in formation of electrostatic latent images. That is, the tandem engine has a problem that a time difference in development exists between a drum arranged upstream and one arranged downstream, and if toner supply timings are adjusted to any drum, a supply target page and toner supply timing do not match each other.